Until recently, most of the 24 d-block transition metals had been used primarily as useful materials for (i) construction and also as tools and containers, etc., (Ti, Zr, Fe and their alloys with V, Cr, Mn, Co, Ni, etc.), (ii) precious and ornamental items (Au, Pt, Ir, Os, Ag, etc.), and(iii) electromagnetic applications (Cu, Nb, Ta, W, Re, etc.). Over the past several decades, their superb properties as chemically useful substances, especially as catalysts for chemical reactions, have been increasingly recognized. “Why are they so useful as catalysts?”

In most cases, their superb catalytic properties may be attributed to one or both of the following two: (1) ability to provide simultaneously both filled nonbonding valence-shell orbitals (one or more) and empty valence-shell orbitals (one or more) within thermally stable species and (2) ability to undergo simultaneously both reduction and oxidation under one set of reaction conditions in one reaction vessel.

A combination of these two properties can be exploited in devising a wide variety of useful catalytic reactions for formation and cleavage of C–C, C–H, C–O and other bonds.